AC motors are frequently used for various drive purposes. These AC motors are started and in part also actively braked again by different mechanical or electronic switching devices. Typical applications of such combinations of starting and braking devices or devices which perform both functions are to found principally in the wood-working industry. Here, for safety reasons, there is a requirement to bring the in part considerable centrifugal masses of the machines, for example milling machines, planing machines and circular saws, to a complete stop within a certain time after switching off. This requirement can for the most part only be met by way of active braking.
Various methods are available for the active braking of AC motors. One frequently used method is DC braking. This can be performed inter alia by way of phase angle control using a thyristor circuit, wherein a pulsating direct current is applied to the AC motor for braking purposes from the three-phase power supply network. This method becomes particularly effective if the circuit contains a freewheeling circuit by way of which the braking current can continue to flow during the negative half wave of the driving voltage. In this case a braking current is injected into the motor with the aid of a first thyristor. At a zero crossing of the voltage present at the first thyristor the freewheeling circuit is closed by way of a second thyristor, which means that the AC motor continues to be braked. The braking of the AC motor is performed periodically by switching the two thyristors.
FIG. 3 shows the timing characteristics of a braking current 20 of an AC motor during the braking process thereof, wherein the AC is braked by way of a thyristor circuit having two thyristors. In this case, the braking current 20 is injected into the three-phase AC motor with the aid of a first thyristor. The timing of the actuation of the first thyristor is visualized by way of the first thyristor curve 22. In this case, the firings 23 of the first thyristor are plotted with respect to the braking current 20 present. Likewise, the timing of the actuation of the second thyristor is visualized by way of the second thyristor curve 21. In this case, the firings 24 of the second thyristor are likewise plotted with respect to the braking current 20 present. While the first and second thyristors are in the fired state 23,24 the respective thyristor is in a conducting state. A braking current is injected into the AC motor by way of the first thyristor while the first thyristor is in the fired state 23.
At a zero crossing of the voltage present at the first thyristor a freewheeling circuit is closed by way of the second thyristor, which means that a portion of the injected braking current continues to be delivered to the AC motor and thus continues braking the AC motor. The braking of the AC motor is performed periodically by switching the two thyristors. In order to terminate the braking process the first and subsequently the second thyristor are no longer fired.
After this point in time, the current continues to flow in the freewheeling circuit until the holding current of second thyristor is undershot. Until the holding current is undershot the second thyristor is conducting and bridges two motor windings of the AC motor, which means that a freewheeling circuit is present. A restart of the AC motor is thus not immediately possible because this would cause a short-circuit through the second thyristor (freewheeling thyristor).
Until the start of the AC motor, it is therefore necessary to observe a waiting time which must be guaranteed by way of a time interlock. Since the minimum waiting time depends on a plurality of parameters, such as AC motor, braking current and temperature for example, it must be designed with an adequate safety margin and can thus be up to several seconds.
A control device for switching on/off and braking an AC asynchronous motor by way of two thyristor switches is known from DE 28 55 330 A1. In order to brake the AC asynchronous motor by way of the thyristor switches, the thyristor switches are actuated for a predetermined braking time by way of a logic unit.